A switching mode power supply means a device for transforming one direct current supply voltage into more than one direct current output voltage. The direct current output voltage has a value larger or smaller than that of the direct current supply voltage.
The switching mode power supply is mainly used in battery power electronic devices such as power electronic devices, particularly a mobile phone and a laptop computer. The power electronic devices have a normal operation mode that consumes a relatively large power and a waiting mode that consumes a relatively small power.
Meanwhile, the power electronic device automatically goes into a waiting mode when a user does not use the power electronic device for a predetermined period of time. When the user uses the power electronic device again, the power electronic device returns to a normal operation mode.
In most power electronic devices, an amount of power consumption in the waiting mode is very small in comparison with an amount of power consumption in the normal operation mode. Recently, regulation on input power in the waiting mode has been gradually reinforced to reduce the amount of power consumption more in the waiting mode.
Generally, a method for reducing output voltage of the switching mode power supply in order to correspond to the regulation or reducing the amount of power consumption in the waiting mode by using individual auxiliary power devices is used.
However, the general method is not preferred since additional constituent elements are required to increase a product price. Also, the general method may show output voltage that is too low to operate the power electronic devices. Accordingly, there is a problem that the amount of power consumption to be reduced is limited.
Meanwhile, when the general switching mode power supply is in a waiting mode, it is inevitable that switching loss incurs in the switching mode power supply in spite of reduction of the amount of power consumption by the reduced output voltage.
In addition, the general switching mode power supply changes a duty cycle of a power switch to compensate changes of power requirement in an output stage and be operated at a regular frequency unrelated to the amount of the supplied power.
Consequently, the power switch of the switching mode power supply performs an on/off switching operation in the waiting mode at the same frequency as that of the normal operation mode. There is a limitation that thus switching operation incurs power loss in the waiting mode by generating a considerable amount of consumption power.
Accordingly, it is essentially required to develop a switching mode power supply that can reduce power consumption more in a waiting mode.